Networking :: Configuring Two Ethernet Ports?

I have a system with one (sometimes two) ethernet ports, that works happily in an old Fedora 5 build. But I can't get it to work on a new Centos 5.4 build. Original system: One dedicated ethernet port on card always connected to the systems dedicated equipment and no external access (the system is the DHCP master for that network). An optional second USB dongle that is a second ethernet port, used for debugging and development. (This is a DHCP client with full conectivity. In /etc/sysconfig/network-scripts I have ifcfg-eth0, ifcfg-eth1 and a route-eth0. Neither of the ifcfg files needs an explicit HWADDR, which means the same ones work for all boxes. And when one needs to be connected to the network all is fine.

The system is being moved to Centos 5.4, most is working with minimal change, but I am having problems with the ethernet ports. If it only has the on board ethernet connected, all is fine. If you have the USB dongle connected things go wrong: This system brings up the USB ethernet first, and tries to assign it to eth0 (which fails), and then brings up the on board ethernet as eth1 (which also fails). I have tried forcing the behaviour of the network by setting the HWADDR(s), but this does not result in the on board coming up as eth0, it comes up as __tmpxxxx as follows:

ifconfig -a
__tmp226406138 Link encap:Ethernet HWaddr 00:80:66:07:A8:63
BROADCAST MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000

[Code]...

Currently the only solution is to unplug the USB dongle through restart and plug in afterwards, and this wont work when the unit is remote and in the field.